1. Field of the Invention
The field of the invention relates to forming depressions in the surface of asphalt roads by impact impression.
2. Description of the Prior Art
Sonic noise alert pattern, (SNAP), are a series of depressions formed in the surface of asphalt. The pattern has the purpose of providing vibration, and therefore noise, when the tires of a vehicle traverse them longitudinally. Road departments use these depressions as a safety device. Longitudinally adjacent the edges of a highway or along the center line which divides the opposing directional traffic flows are common locations of placement. They act to alert a driver that his or her vehicle has extended beyond the normal driving surface. Beyond this normal driving surface many dangerous conditions exist for a vehicle traveling near the posted speed limit. These dangers include dirt or gravel shoulders, guardrail barriers, signs, mailboxes, intersecting roadways or driveways and disabled vehicles.
The various specifications for the placement and physical dimensions of the individual depressions can vary from state to state and even within a particular state. A common size and placement, used only for illustration and not limitation, places the individual depressions twelve inches apart from center of one depression to center of the adjacent depressions. The measurements of the individual depressions being seven inches from back trailing edge to front leading edge with a depth, at the deepest point, of one half inch and a lateral length across of sixteen inches. These specifications result in five inches of untreated surface between each set of adjacent depressions. Therefore, the above specifications would require fifty-two hundred and eighty depressions per mile. Limited access highways and rural roads are likely locations for SNAP depressions to be installed due to the fatigue that a driver experiences during extended driving on such roads.
A recent innovation in the specifications for the installation of SNAP depressions requires a skip pattern be incorporated within the series. One example of such a series has a repetitive cycle of eight depressions spaced as detailed above followed by an untreated area equal to the normal placement of four depressions. Such installation affords reasonable coverage of a highway while reducing the expense of installation.
Various attempts have been made to provide a machine capable of quickly, accurately, consistently and precisely installing SNAP depressions. Your applicants are aware of only one method currently known for forming such depressions. This method requires a rotary milling head to physically remove a desired amount of asphalt to form each depression.
Several machines exist capable of milling such depression utilizing a rotary cutting head. A plunge cut can be made from a stationary position to form one or more depressions per cut. A second method involves continuously advancing the cutting head while regulating the raising and lowering actions.
The primary disadvantage of milling is that material is removed by the cutting operation. Overcutting is where the rotary cutting head is allowed to penetrate further than desired. Occasionally this results in extending through the asphalt layer into the bedrock. When material is removed, water gains entry and over time erodes and weakens the rock layer. This can result in premature failure of the road.
Therefore milling has been less efficient than desired. As such, it may be appreciated that there continues to be a need for a machine that can consistently form depressions having precise placement and precise dimensions without requiring removal of material from the road surface. The present invention substantially fulfills these needs.